Display devices comprising green-emitting quantum dots and red KSF phosphor

ABSTRACT

LED devices emitting white light comprise a blue-emitting LED, green-emitting quantum dots (QDs) and red-emitting K 2 SiF 6 :Mn 4+  (KSF) phosphor. A backlight unit (BLU) for a liquid crystal display (LCD) comprises one or more blue-emitting LEDs and a polymer film containing green-emitting QDs and KSF phosphor. The QDs and/or KSF phosphor may be encapsulated in beads that provide protection from oxygen and/or moisture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 15/337,557filed on Oct. 28, 2016, which claims the benefit of U.S. ProvisionalApplication No. 62/249,595 filed on Nov. 2, 2015, the contents of whichare hereby incorporated by reference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to materials and processes for theproduction of display devices. More specifically, the invention relatesto backlight units for display devices comprising quantum dots andK₂SiF₆:Mn⁴⁺ (KSF) phosphor.

2. Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98

Quantum dots (QDs) have been used in the backlight unit (BLU) of liquidcrystal displays (LCDs) to improve the color gamut compared toconventional LCD backlight units comprising a blue LED and aconventional rare earth phosphor. The color gamut is compared tostandards in the Commission Internationale de l'Eclairage (CIE) 1931color space. Such standards include the National Television SystemCommittee 1953 (NTSC 1953) and the Digital Cinema Initiatives-P3(DCI-P3) color triangles. In 2012, the International TelecommunicationsUnion recommended a new standard for ultra-high definition television(UHDTV), known as Rec. 2020. The color triangle for Rec. 2020 issignificantly larger than the NTSC 1953 and DCI-P3 color triangles,covering 76% of the color space seen by the human eye.

QDs have been used to produce BLUs with good overlap with the NTSC 1953and DCI-P3 color space. However, achieving good overlap with Rec. 2020is a challenge, in part owing to the full-width at half-maximum (FWHM)of the spectral emission peaks of the QDs.

A recent presentation suggested that, to achieve greater than 90%overlap with the Rec. 2020 color space, green QDs emitting at 526 nmwith a FWHM of 30 nm and red-emitting QDs emitting at 640 nm with a FWHMof 30 nm would be required. [J. Hartolve, J. Chen, S. Kan, E. Lee and S.Gensler, Bringing Better Pixels to UHD with Quantum Dots. Presented atQuantum Dots Forum, 18 Mar. 2015, San Francisco] FWHM values in thisregion may be achieved using cadmium-based QDs, such as CdSe. However,in the EU, the Restriction of the Use of Certain Hazardous Substances(RoHS) Directive limits the amount of cadmium (and other heavy-metals)that may be used in electrical and electronic equipment. Similarlegislation is being adopted throughout the world. Thus, there is a needto develop display devices comprising heavy-metal-free QDs, i.e. QDswithout the incorporation of toxic elements such as Pb, Hg or Cd, suchas Group III-V-based QDs, which are compliant with RoHS and other healthand safety legislation. Such heavy-metal-free QDs may include GroupIII-V-based materials (such as InP), as well as alloys of e.g. GroupIII-V materials alloyed with other elements, e.g. InPZnS (as describedin U.S. Publication No. 2014/0264172,). In Group III-V-based materials,the quantum confinement effects are stronger than those in Group III-Vmaterials (including CdSe). This leads to a larger change in emissionwavelength from a given change in particle size, and consequently abroader FWHM is displayed by Group III-V materials than Group II-Vmaterials. FWHM values are typically broader for red-emittingheavy-metal-free QDs than green-emitting heavy-metal-free QDs.

One potential means for providing a broad color gamut is to combinegreen-emitting QDs with a narrowband-emitting red phosphor, such asK₂SiF₆:Mn⁴⁺ (KSF). KSF has four emission maxima at 613 nm, 631 nm, 636nm and 648 nm, each with a FWHM<5 nm. [P. Pust, P. J. Schmidt and W.Schnick, Nat. Mater., 2015, 14, 454] Zhang et al. recently reported awhite LED comprising a blue GaN LED, green-emitting CH₃NH₃PbX₃ (X=Br, I,Cl) QDs and a red-emitting KSF phosphor. [F. Zhang, H. Zhong, C. Chen,X. G. Wu, X. Hu, H. Huang, J. Han, B. Zhou and Y. Dong, ACS Nano, 2015,9, 4533] The device covered ˜130% of the NTSC color space, with 96%overlap. To form the LED device, KSF powder was blended with siliconegel and left to cure for minutes. CH₃NH₃PbBr₃ QDs were dissolved inchloroform with polymethylmethacrylate. The layer of silicone gel withKSF was painted onto the surface of a blue GaN LED chip, followed bycasting a layer of QDs in PMMA. Though the study demonstrates that alarge color gamut may be achieved by combining green QDs with a red KSFphosphor, the CH₃NH₃PbX₃ QDs are not currently commercially availableand contain lead, another heavy-metal subject to RoHS regulations.Further, the on-chip configuration may create thermal stability issuesdue to the close proximity of the QDs to the LED chip, resulting inshort device lifetimes.

Manganese-activated fluoride complex phosphors are phosphors having Mn⁴⁺as an activator and a fluoride complex salt of an alkali metal, anamine, or an alkaline earth metal as a base crystal. The coordinationcenter in the fluoride complex that forms the base crystal is atrivalent metal (B, Al, Ga, In, Y, Sc or a lanthanoid), a tetravalentmetal (Si, Ge, Sn, Ti, Zr, Re, Hf), or a pentavalent metal (V, P, Nb orTa) with 5 to 7 fluorine atoms coordinated around the coordinationcenter.

A preferred Mn⁴⁺-activated fluoride complex phosphor is A₂MF₆:Mn havinga hexafluoro complex salt of an alkali metal as a base crystal (where Ais selected from the group consisting of Li, Na, K, Rb, Cs and NH₄, andM is selected from the group consisting of Ge, Si, Sn, Ti and Zr).Particularly preferred is a Mn⁴⁺-activated fluoride complex phosphorwhere A is K (potassium) or Na (sodium), and M is Si (silicon) or Ti(titanium)—for example, K₂SiF₆:Mn⁴⁺ (KSF).

There is a need in the art to provide a means to integrateheavy-metal-free QDs and KSF phosphor into display devices, whereby theoptical stability of the QDs is maintained.

BRIEF SUMMARY OF THE INVENTION

Herein, a method is disclosed for combining a blue-emitting LED,green-emitting QDs, and red-emitting KSF phosphor to produce white lightwith a wide color gamut, in such a way that the optical properties ofthe QDs are not deteriorated by exposure to heat. LED devices comprisinga blue-emitting LED chip, green-emitting QDs and red-emitting KSFphosphor are also disclosed.

In some embodiments, the QDs and/or the KSF phosphor are incorporatedinto polymer beads. In some embodiments, the QDs and the KSF phosphorare incorporated into the same bead. In alternative embodiments, the QDsand the KSF phosphor are incorporated into separate beads. In otherembodiments, the QDs and the KSF phosphor are incorporated into a“core/shell bead” having a central core comprising a first materialsurrounded by a shell comprising a second material. The KSF phosphor maybe incorporated into the core of the bead and QDs into the shell of thebead. Alternatively, the QDs may be incorporated into the core of thebead and KSF phosphor into the shell of the bead.

In some embodiments, the QDs and/or the KSF phosphor are deposited intothe cup portion of an LED package.

In some embodiments, the QDs and/or the KSF phosphor are incorporatedinto a film that is positioned remotely from the LED chip.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a cross-sectional view of a polymer bead comprising greenquantum dots and KSF phosphor particles.

FIG. 2 is a cross-sectional view of two polymer beads; one beadcontaining KSF phosphor and the other bead containing green quantumdots.

FIG. 3 is a cross-sectional view of a core/shell polymer bead comprisinga core comprising KSF phosphor particles and a shell comprising greenquantum dots.

FIG. 4 is a cross-sectional view of a core/shell polymer bead comprisinga core comprising green quantum dots and a shell comprising KSF phosphorparticles.

FIG. 5 is a cross-sectional view of an LED comprising an LED chip and anLED cup filled with KSF phosphor and green quantum dot polymer beads.

FIG. 6 is a cross-sectional view of an LED comprising an LED chip and anLED cup filled with KSF phosphor, and a remote film comprising greenquantum dots.

FIG. 7 is a cross-sectional view of an LED comprising an LED chip and anLED cup filled with KSF phosphor polymer beads and green quantum dotpolymer beads.

FIG. 8 is a cross-sectional view of an LED comprising an LED chip and anLED cup filled with polymer beads comprising KSF phosphor and greenquantum dots.

FIG. 9 is a cross-sectional view of an LED comprising an LED chip and anLED cup filled with polymer beads comprising KSF phosphor, and a remotefilm comprising green quantum dots.

FIG. 10 is a cross-sectional view of an LED and a remote film comprisingpolymer beads comprising KSF phosphor and polymer beads comprising greenquantum dots.

FIG. 11 is a cross-sectional view of an LED and a remote film comprisingpolymer beads comprising KSF phosphor and green quantum dots.

FIG. 12 is a cross-sectional view of an LED and a remote film comprisingKSF phosphor particles and green quantum dots.

FIG. 13 is a simulated spectrum of a film containing green-emittingquantum dots and KSF phosphor.

FIG. 14 is a simulated spectrum of a film containing green-emittingquantum dots and red-emitting quantum dots.

FIG. 15 is a normalized emission spectrum of polymer beads containingKSF phosphor.

FIG. 16 is a normalized emission spectrum of polymer beads containinggreen-emitting quantum dots.

FIG. 17 is a normalized emission spectrum of beads containing bothgreen-emitting quantum dots and KSF phosphor.

FIG. 18 is an exploded view of a liquid crystal display (LCD) equippedwith a quantum dot backlight unit.

DETAILED DESCRIPTION OF THE INVENTION

A blue-emitting LED may be combined with green-emitting QDs, andred-emitting KSF phosphor to produce white light with a wide color gamutin such a way that the optical properties of the QDs are not adverselyaffected by exposure to heat.

In a simulation, a combination of green-emitting quantum dots with KSFphosphor was shown to provide similar color performance relative to boththe DCI and Rec. 2020 standards as compared to a combination ofgreen-emitting and red-emitting quantum dots. However, the combinationof green-emitting quantum dots and KSF phosphor was photometricallybrighter. The simulation data is shown in Table 1.

TABLE 1 Green Red Green PL, FWHM, Red PL, FWHM, DCI Coverage Rec2020Coverage nm nm nm nm Lum CIE 1931 CIE1976 CIE 1931 CIE1976 536 42 KSF11.30 90.9% 96.2% 70.5% 78.8% 536 38 KSF 11.33 91.2% 96.2% 71.4% 79.7%536 42 640 60 9.08 90.9% 96.0% 68.7% 77.5% 536 38 640 60 9.07 91.2%96.1% 69.6% 78.3% 536 38 635 55 9.64 91.3% 96.1% 69.2% 77.5% 536 38 64555 8.44 91.2% 96.1% 71.6% 81.1%

A simulated spectrum of a film containing green-emitting quantum dotsand KSF phosphor [excited with blue light?] is shown in FIG. 13 and asimulated spectrum of a film containing green-emitting quantum dots andred-emitting quantum dots is shown in FIG. 14. Also shown in FIGS. 13and 14 are the transmission spectra of the red, green, and blue colorfilters used in a typical LCD television set.

In certain embodiments, the QDs and/or the KSF phosphor are incorporatedinto polymer beads. The incorporation of QDs into polymer beads isdescribed in U.S. Pat. No. 7,544,725 “Labelled Beads;” U.S. Pat. No.8,859,442 “Encapsulated Nanoparticles;” U.S. Pat. No. 8,847,197“Semiconductor Nanoparticle-Based Materials;” and U.S. Pat. No.8,957,401 “Semiconductor Nanoparticle-Based Materials;” and in publishedpatent applications: U.S. Pub. No. 2014/0264192 “Preparation of QuantumDot Beads Having a Silyl Surface Shell” and U.S. Pub. No. 2014/0264196“Multi-Layer-Coated Quantum Dot Beads,” the disclosures of which arehereby incorporated by reference in their entireties. Similar techniquesmay be used to incorporated phosphor particles into polymer beads. Theincorporation of phosphor particles, such as KSF phosphor particles, isdescribed in U.S. Pat. No. 7,323,696 “Phosphor Particle Coded Beads.”

In some embodiments, green-emitting QDs and red-emitting KSF phosphormay be incorporated into the same bead, as illustrated in FIG. 1.Incorporating the QDs and the KSF phosphor into the same bead may beadvantageous for ease of processing.

In alternative embodiments, green-emitting QDs and red-emitting KSFphosphor may be incorporated into separate beads, as illustrated in FIG.2. Incorporating the QDs and the KSF phosphor into separate beads canoffer an effective means of color mixing, since suitable ratios ofgreen- and red-emitting beads may be combined to produce a desired whitepoint in the CIE 1931 color space.

In yet other embodiments, green-emitting QDs and red-emitting KSFphosphor may be incorporated into a core/shell bead—i.e., a bead havinga central core comprising a first material surrounded by a shellcomprising a second material. KSF phosphor may be incorporated into thecore of the bead and green-emitting QDs into the shell of the bead, asshown in FIG. 3, or green-emitting QDs may be incorporated into the coreof the bead and KSF phosphor into the shell of the bead, as shown inFIG. 4.

In some embodiments, the QDs and/or the KSF phosphor are deposited intothe cup portion of an LED package. Techniques to deposit QDs andphosphor particles into LED cups are known in the art and can includecombining the particles with an optically-transparent resin, depositingthe resin in an LED cup, and allowing the resin to cure. For example,the incorporation of QDs into an acrylate resin and subsequentdeposition into LED cups is described in U.S. Pub. No. 2013/0105839“Acrylate Resin for QD-LED” the disclosure of which is herebyincorporated by reference in its entirety. Examples of suitable resinsinclude, but are not restricted to, acrylates, silicones and epoxies.

In some embodiments, the QDs and/or the KSF phosphor are incorporatedinto a film that is positioned remotely from the LED chip. This isparticularly advantageous for separating the QDs from the LED chip,which prevents thermal degradation of the QDs due to heat emitted by theLED. Techniques to incorporate QDs into a resin matrix are known in theprior art, for example, as disclosed in U.S. Patent Pub. No.2015/0047765 “Quantum Dot Films Utilizing Multi-Phase Resins;” U.S.Patent Pub. No. 2015/0275078 “Quantum Dot Compositions;” and U.S. PatentPub. No. 2015/0255690 “Methods for Fabricating High Quality Quantum DotPolymer Films.” Similar techniques may be used to incorporate KSFphosphor particles into resin matrices.

In some embodiments, the QDs and/or the KSF phosphor are firstincorporated into polymer beads, which are subsequently incorporatedinto a polymer film. The formation of a polymer film incorporating QDbeads is described in U.S. Patent Pub. No. 2013/0075692 “SemiconductorNanoparticle-Based Light-Emitting Materials which is hereby incorporatedby reference in its entirety.

In alternative embodiments, the QDs and/or the KSF phosphor areincorporated directly into a polymer or resin matrix and processed intoa film.

In one particular embodiment, illustrated in FIG. 5, a white-emittingLED comprises a blue LED chip and an LED cup filled with KSF phosphorand green-emitting quantum dot polymer beads. By incorporating the QDsinto beads, the QDs are protected from thermal degradation resultingfrom heat generated by the LED chip.

In another embodiment, as shown in FIG. 6, a white-emitting LEDcomprises a blue-emitting LED chip, an LED cup filled with red-emittingKSF phosphor, and a remote polymer film comprising green-emitting QDs.The remote location of the QD film, such that it is not in directcontact with the LED chip, protects the QDs from thermal degradation bythe LED.

In a further embodiment, as shown in FIG. 7, a white-emitting LEDcomprises a blue-emitting LED chip and an LED cup filled withred-emitting KSF phosphor polymer beads and green-emitting QD polymerbeads.

In a further embodiment, illustrated in FIG. 8, a white-emitting LEDcomprises a blue-emitting LED chip and an LED cup filled with polymerbeads comprising red-emitting KSF phosphor and green-emitting QDs. TheKSF phosphor and QDs may be randomly distributed throughout the beads,as illustrated in FIG. 1, or may be formed into core/shell beads, asshown in FIGS. 3 and 4.

In another embodiment, as shown in FIG. 9, a white-emitting LEDcomprises a blue-emitting LED chip, an LED cup filled with polymer beadscomprising red-emitting KSF phosphor, and a “remote film” comprisinggreen-emitting quantum dots—i.e., a film physically separated from theLED package.

In a further embodiment, as shown in FIG. 10, a white-emitting LEDcomprises a blue-emitting LED chip, and a remote polymer film comprisingred-emitting KSF phosphor contained in polymer beads and polymer beadscomprising green-emitting QDs.

In a further embodiment, illustrated in FIG. 11, a white-emitting LEDcomprises a blue-emitting LED chip, and a remote polymer film comprisingpolymer beads comprising red-emitting KSF phosphor and green-emittingQDs. The KSF phosphor and QDs may be randomly distributed throughout thebeads, as shown in FIG. 1, or may be formed into core/shell beads, asshown in FIGS. 3 and 4.

In a further embodiment, as shown in FIG. 12, a white-emitting LEDcomprises a blue-emitting LED chip, and a remote polymer film comprisingred-emitting KSF phosphor particles and green-emitting QDs.

In alternative embodiments, another narrowband phosphor may besubstituted for the KSF phosphor.

The invention has particular application in the backlight units used inliquid crystal displays. A liquid crystal display is an electronicallymodulated optical device made up of any number of segments controlling alayer of liquid crystals arrayed in front of a light source (backlight)or reflector to produce images in color or monochrome. A backlight is aform of illumination used in liquid crystal displays (LCDs). BecauseLCDs do not themselves produce light (unlike, for example, CRT displaysand LED displays), they need an illumination source (ambient light or aspecial light source) to produce a visible image. Backlights mayilluminate the LCD from the side or back of the display panel.

An exemplary QD-based backlight unit is shown in FIG. 18. Theillustrated BLU includes a brightness enhancement film (BEF) and aliquid crystal matrix (LCM). The QD-containing layer is shown displacedlaterally from its actual position in order to permit visualization ofthe LEDs. In an embodiment of the invention, the “QD” layer is a polymerfilm comprising green-emitting quantum dots and KSF phosphor. Thequantum dots and/or the KSF phosphor may be contained within beads thatprovide protection from heat, oxygen and/or moisture.

EXAMPLES

Method for Making QD/Polymer- and KSF-Containing Beads

Preparation of Polyvinyl Alcohol (PVOH) Solution:

Aqueous PVOH solution was prepared by dissolving PVOH in distilled waterand stirring overnight. For further dissolution of the undissolved PVOHpart the whole solution was heated at 70° C. for 3-4 hr. For beadsynthesis, the PVOH solution was filtered to remove any undissolvedpolyvinyl acetate and/or dust particles.

Standard Resin Preparation

General Procedure

First, the required amount of QD dot solution was transferred to a glassvial containing a stirring bar under an inert atmosphere. Toluene wasremoved from the QD dot solution using reduced pressure with continuousstirring. Once all traces of visible solvent were removed, the residuewas heated to 40° C. for 45 minutes under vacuum to ensure that alltraces of residual solvent were removed. The stock solution was preparedby adding degassed lauryl methacrylate (LMA) and the cross-linkertrimethylolpropane trimethacrylate (TMPTM) to the degassedphotoinitiator(s) and stirring in the dark to ensure completedissolution of the photoinitiator(s). Then, the required amount of thestock solution was added to the dry QD residue under an inert atmosphereand in dark conditions. The entire resin solution was stirred overnightto ensure complete dispersion of the QDs.

Bead Synthesis

General Procedure

First, the required amount of TWEEN® 80 nonionic, sorbitan estersurfactant [CRODA AMERICAS LLC, WILMINGTON DELAWARE 19801] was added tothe reaction vessel followed by the required amount of PVOH solution andstirred for at least 30 minutes for complete dissolution of thesurfactant into the PVOH solution. The whole solution mixture wasdegassed for a few hours by applying a vacuum/nitrogen cycle. Aftercompletion of the degassing procedure, the reaction vessel was placedinside a rig equipped with a four UV-LED head. Then, the required amountof QD resin solution was injected into the PVOH/surfactant solutionunder constant stirring. After a certain stirring time, the beadpolymerization was started by activating the UV light source for severalminutes (depending on the reaction conditions). After thepolymerization, the beads were washed twice with cold distilled waterthen once with a water/acetonitrile (50/50 volume ratio) mixture thentwice with only acetonitrile. Usually, the beads were washed in acentrifuge machine. After washing, the beads were dried under vacuum forseveral hours or overnight.

A typical bead synthesis was carried out as follows:

KSF Beads

First, 0.4 g of TWEEN 80 surfactant was added to 10 mL of a 4% PVOHsolution and stirred for 30 minutes for complete dissolution of thesurfactant into the PVOH solution and degassed for a few hours byapplying a vacuum/nitrogen cycle. For KSF resin preparation, first therequired amount of KSF was degassed and then the required amount ofdegassed LMA/TMPTM (1:0.154 volume ratio with 1% IRGACURE® 819photo-initiator [BASF SE COMPANY CARL-BOSCH-STR. 38 LUDWIGSHAFEN,GERMANY 67056]) resin was added from the LMA/TMPTM stock solution. TheKSF resin solution was stirred overnight under an inert atmosphere andin dark conditions. The next day, 0.5 mL of the KSF resin solution wasinjected into 10 mL of degassed PVOH/surfactant solution under constantstirring at 800 rpm with a cross magnetic stirrer bar and under anitrogen atmosphere. After 10 minutes of stirring, bead polymerizationwas started by activating the UV light source for 10 minutes. Theresulting beads were washed by the method described above and driedunder vacuum for several hours.

Green QD Beads

First, 0.4 gram of TWEEN 80 surfactant was added to 10 mL of a 4% PVOHsolution and stirred for 30 minutes for complete dissolution of thesurfactant into the PVOH solution and degassed for a few hours byapplying a vacuum/nitrogen cycle. For green QD resin preparation, firsta QD solution was dried under vacuum. Then, 3 grams of degassedLMA/TMPTM (1/0.154 volume ratio with 1% IRGACURE 819) resin was addedfrom the LMA/TMPTM stock solution and stirred overnight under an inertatmosphere and in dark conditions. The whole QD resin solution wasstirred overnight to ensure complete dispersion of the QDs. 0.5 mL ofthe QD resin solution was injected the next day into 10 mL of thedegassed PVOH/surfactant solution under constant stirring at 800 rpmwith a cross magnetic stirrer bar and under an N₂ atmosphere. After 10minutes of stirring, bead polymerization was started by activating theUV light source for 10 minutes. The resulting beads were washed by themethod described above and dried under vacuum for several hours.

QD/KSF White Beads

First, 0.4 g TWEEN 80 surfactant was added to 10 mL of a 4% PVOHsolution and stirred for 30 minutes to obtain complete dissolution ofthe surfactant into the PVOH solution and degassed for a few hours byapplying a vacuum/nitrogen cycle. For QD/KSF white bead synthesis, firstKSF and green QD resin were prepared separately following the sameprocedure described above. Then, a quantity of KSF and green QD resinwere mixed together for 2 hours to produce “white” KSF/QD beads—i.e.,beads that provide white light when excited by and mixed with bluelight.

These and other advantages of the present invention will be apparent tothose skilled in the art from the foregoing specification. Accordingly,it is to be recognized by those skilled in the art that changes ormodifications may be made to the above-described embodiments withoutdeparting from the broad inventive concepts of the invention. It is tobe understood that this invention is not limited to the particularembodiments described herein and that various changes and modificationsmay be made without departing from the scope of the present invention asliterally and equivalently covered by the following claims.

What is claimed is:
 1. A light-emitting device comprising: a blue lightemitting diode (LED); one or more particles of K₂SiF₆:Mn⁴⁺ (KSF)phosphor optically coupled to the blue LED; and a plurality of quantumdots optically coupled to the blue LED, wherein the one or moreparticles of KSF phosphor and the plurality of quantum dots are in afilm remote from a package containing the blue LED.
 2. Thelight-emitting device recited in claim 1 wherein the quantum dots arecontained within at least one first type polymer bead and the one ormore particles of KSF phosphor are contained within at least one secondtype polymer bead.
 3. The light-emitting device recited in claim 1wherein the quantum dots are contained within a plurality of polymerbeads, each polymer bead comprising one or more particles of KSFphosphor and a plurality of quantum dots.
 4. The light-emitting devicerecited in claim 3 wherein the beads comprise a polymer selected fromthe group consisting of acrylates, silicones, and epoxies.
 5. Thelight-emitting device recited in claim 1 wherein the quantum dots aregreen-emitting quantum dots.
 6. The light-emitting device recited inclaim 5 wherein the light emitted by the device comprises a mixture ofgreen light from the green-emitting quantum dots together with red lightemitted by the particles of KSF and blue light from the LED.
 7. A beadcomprising an optically-transparent resin and a plurality of KSFphosphor particles embedded in the resin.
 8. The bead recited in claim 7further comprising: a plurality of quantum dots embedded in the resin.9. A bead comprising a central core portion surrounded by a shellportion comprising an optically-transparent resin and a plurality of KSFphosphor particles embedded in the shell portion resin.
 10. The beadrecited in claim 9 further comprising a plurality of quantum dots in thecore portion.
 11. A bead comprising: a central core portion comprisingan optically-transparent resin having a plurality of KSF phosphorparticles embedded therein; and a shell portion surrounding the centralcore portion and comprising a plurality of quantum dots.
 12. A methodfor generating white light comprising: providing a blue light-emittingdiode (LED); providing one or more particles of K₂SiF₆:Mn⁴⁺ (KSF)phosphor optically coupled to the blue light-emitting diode (LED); andproviding a plurality of green-emitting quantum dots optically coupledto the blue light-emitting diode (LED), wherein the one or moreparticles of KSF phosphor and the plurality of green-emitting quantumdots are remote from a package containing the LED.
 13. A backlight unitfor a liquid crystal display comprising: a blue light-emitting diode(LED); a polymer film optically coupled to the blue light-emitting diode(LED) and containing one or more particles of K₂SiF₆:Mn⁴⁺ (KSF) phosphorand a plurality of quantum dots, wherein the polymer film is remote froma package containing the LED.
 14. The backlight unit recited in claim 13wherein the one or more particles of KSF phosphor are contained withinat least one polymer bead.
 15. The backlight unit recited in claim 13wherein the quantum dots are contained within at least one polymer bead.16. The backlight unit recited in claim 13 wherein the quantum dots arecontained within at least one first type polymer bead and the one ormore particles of KSF phosphor are contained within at least one secondtype polymer bead.
 17. The backlight unit recited in claim 13 whereinthe quantum dots are contained within a plurality of polymer beads, eachpolymer bead comprising one or more particles of KSF phosphor and aplurality of quantum dots.
 18. The backlight unit recited in claim 17wherein the beads comprise a polymer selected from the group consistingof acrylates, silicones, and epoxies.
 19. The backlight unit recited inclaim 13 wherein the quantum dots are green-emitting quantum dots. 20.The backlight unit recited in claim 13 wherein the quantum dots areheavy metal-free quantum dots.